The Clemens patent discloses a video disc for use with a playback system of the variable capacitance type. In one configuration of the Clemens' system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths of approximately 2.6 micrometers and groove depths of about 0.5 micrometers may be used. During playback, capacitive variations between a conductive electrode on a stylus and a conductive property of the disc record are sensed to recover the prerecorded information.
In accordance with the Clemens' format, the video information may be recorded as relatively short (e.g., 0.6-1.6 micrometers) relief variations along the length of the spiral groove. Illustratively, the method of recording may be of a type shown in U.S. Pat. No. 4,044,379 to J. B. Halter. Pursuant to the Halter method, an electromechanically-driven stylus (e.g., of diamond) having a trangular shape, responsive to a combined video and audio signal, records relatively short geometric variations, representative of the time variations of the signal, on a surface of a metal substrate. After the electromechanical recording operation, the recorded surface of the metal substrate has a relief pattern corresponding to that which is desired in the final record. In the replicating process, masters are made from the substrate. Molds are then made from the masters and stampers are made from the molds. The stampers are used in the process of pressing a vinyl record having the desired relief pattern.
During each of the above-identified record manufacturing processes, various kinds of defects can develop which may affect the record groove quality and which are difficult to detect in view of the fineness of the groove structure typically employed in a video disc (e.g., 10,000 groove convolutions per inch [4000 convolutions per cm]).
Because of the very small dimensions of the signals recorded on the video disc, very small defects can disturb the playback of a disc. Many defects which cause trouble in playback are difficult to see when viewed under normal lighting or when observed through a microscope.
Defect detection systems utilizing collimated (laser) light beams to illuminate grooved surfaces are described in U.S. Pat. No. 4,030,835 issued to A. H. Firester et al. on June 21, 1977; copending U.S application Ser. No. 258,759, filed Apr. 29, 1981 by Southgate et al now U.S. Pat. No. 4,395,122; and copending U.S. patent application Ser. No. 155,989, fi1ed May 30, 1980 by W. R. Roach now U.S. Pat. No. 4,352,564. These systems utilize certain diffraction orders reflected from the incident collimated light to identify defects of various kinds. U.S. Pat. No. 3,460,841, issued to K. G. Caldwell on Aug. 12, 1969, describes a system for testing a grooved record utilizing a point source of non-collimated light to illuminate the grooved record surface to effect diffracted reflections. The diffracted reflections are viewed to identify damage to the grooves.
None of the above described systems for inspecting grooved surfaces are useful for inspection purposes to identify quickly and easily defects in the topography of grooved surfaces defining a diffraction grating.
Moreover, optical discs having closely spaced spiral or circular information tracks formed of pits or bumps in the surface also form diffraction gratings and also may have defects in the topography that are not quickly and easily identified. U.S. Pat. No. 4,300,143 by A. E. Bell et al., describes an optical disc with the associated laser recording and playback apparatus. See also "Technology Focus: Video discs--a triquetral marketplace," Electronic Engineering, Sept. 1980, pp. 129, et seq.